A wide variety of detection systems have been employed to sense proper or improper conditions of sheet feed along a use path, e.g. the path of original or copy sheets feed in copier apparatus, the feed path of documents to be scanned by input scanner devices or the feed path of print sheets within printing machines. Conditions which have been detected by such systems include (i) position error (e.g. longitudinal error, lateral position error and skew error), (ii) double sheet feed or incorrect sheet thickness and (iii) improper sheet type (e.g. transparency vis a vis opaque sheet). The type and sensitivity of the detection system utilized depends on the kind of machine in which the system is used and the error condition that is to be detected.
U.S. Pat. Nos. 3,230,519; 3,591,922; 3,646,372 and 4,258,326 disclose the use of capacitive sensing devices to detect the presence or absence of dielectric objects, a variation in thickness because of a web splice and the existence of a double sheet feed. Capacitive detection devices have heretofore been employed in system configurations that enable detection of abrupt changes in feed material thickness but have not been used for accurate positioning detection.
U.S. Pat. No. 4,738,442 discloses the use of two adjustable lead edge photosensor elements which detect proper positioning of a print sheet feed into the grippers of a printer feed table. Prior art photodetection devices of the kind mentioned above can all function well in expected nominal conditions; however, difficulties can arise when deviations from the nominal occur. For example such systems can experience errors when the light reflected from the sensed sheet varies, e.g. because of sheet reflectance variation or a decrease in emitted light.
In many printing, copying and scanning applications, it is important that fed sheets be very accurately positioned. For example, in situations where different color image portions are to be placed on print or copy sheets during successive passes, accurate positioning is necessary to assure proper register of the different color image portions. When data is to be printed in proper register on a preprinted form or personalized printing is to be added to advertising literature, accurate positioning of the blank form or advertising signature sheet is important. In systems where input scanners merge data from separate input documents, accurate relative location information can require accurate positioning of the input and output sheets.